(copyright) 2003 The Hartman Company. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR xc2xa71.71(d), (e).
This invention pertains to the field of heating, ventilation, and air conditioning and, more specifically, to an improved method for repositioning electric actuators of the type employed in modulating valves that control fluids, or other modulating equipment that employ electric actuators.
This invention pertains to systems in which the flow of a fluid, such as water or air, is intended to be regulated so as to meet a particular condition, such as a variable thermal load or other condition that requires continuously regulated, variable positioning. Typical applications include fluid distribution systems in which a continuously regulated flow is required through multiple devices that serve multiple thermal loads, each with the capacity to continuously adjust the flow.
Such a system is shown in FIG. 1. In FIG. 1, a fluid distribution system serves a number of loads, A through N, each of which employs a modulating valve 111A through 111N and a modulating valve actuator 116A through 116N, each operated by a controller 120A through 120N that is capable of sending a valve position command signal through an electrical connection 117A through 117N. The purpose of the valve position command signal is to adjust the fluid flow through the load to meet the current load requirements. A load can be any end device or equipment that is served by the fluid distribution system.
The flow of fluid (water, for example) through each outlet is regulated to control the flow of chilled or heated water through a coil 114A through 114N, which conditions air that is being circulated from space through the coil by a fan 118A through 118N. For water distribution systems, the valve can be installed on the inlet to the load or the outlet from the load, but is usually installed on the outlet as shown in FIG. 1 to reduce the noise of the water in the coil, which would be transferred to the air. Positioning of the valve on the load outlet is also preferred to reduce the temperature extremes to which the valve is exposed. In this application, the opening of each water control valve is modulated to maintain a specific temperature of air being supplied to the space as measured by a supply air temperature sensor 122A through 122N or to maintain some other parameter that requires continuously adjustable flow through the load. The temperature of the space is often regulated by another temperature sensor 124A through 124N that is located in an enclosed area or space 128A through 128N, or as required to sense the temperature and, therefore, load condition of the space. When employed to modulate heating or cooling water for commercial processes, the configuration of FIG. 1 may vary slightly. The method of obtaining a position command signal to position a load valve for space or process thermal control is well known and not a subject of this invention.
Such a fluid distribution system may be quite extensive, serving an entire building, or sometimes multiple buildings. To ensure that an adequate flow of fluid, which may be water, air, or another fluid, is supplied at all times to all loads, the prime mover, in this instance one or more electrically driven centrifugal pumps 150 that circulate water through a closed circuit that is heated or cooled by a heat exchanger 158 or some other means, is often operated by a variable speed drive 154. Pump motor speed is adjusted by a digital or other type controller 156 to maintain a differential pressure between a water supply header 162 and water return header 166, using a differential pressure sensor 168. The differential pressure sensor is typically installed at, or very near, the end of the distribution system to ensure the design fluid pressure is maintained at a minimum setpoint value throughout the distribution circuit.
FIG. 1 is typical of the type of hydronic pumping systems that are employed to distribute heated or chilled water to systems within buildings, or to multiple buildings in a campus type arrangement. The distribution system in FIG. 1 serves a total of xe2x80x9cNxe2x80x9d loads, but only the first (A) and the last (N) are shown. Flow to each device is regulated by some thermal sensing means linked to a controller that sends position command signals to the actuator that operates a modulating valve. The pump(s) is (are) controlled by one or more differential pressure sensor(s) at or near the end of the distribution pumping main(s). This method of regulating pump operation is also well known and not a subject of this invention.
In such systems, each valve is modulated within a range to meet the flow capacity of the device to which it is connected. For example, if, as in FIG. 1, the device is a heating or cooling coil in an air supply system for comfort conditioning, the valve may be modulated to maintain a specific air temperature into the space served by the device. As the load in the space changes due to loads external or internal to the space, the space temperature sensor senses the change in space temperature and control logic is employed to adjust the valve in order to change the temperature of the air supply to the space. In other applications, the control valve may be modulated to maintain parameters other than air discharge or space temperature. In present art, the control devices 120A through 120N send periodic position command signals to the respective device actuators 116A through 116N to maintain each load with respect to a controlled variable, in the case of this example, a supply air temperature to the space or area where thermal condition is being controlled. The frequency of position command signals varies among products and applications and there is no standard for setting how often a reposition of a valve or other device is made. Some individuals recommend reposition intervals of less than several seconds. For example, Burt Rishel, a Fellow of the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), recommends an interval of less than two seconds for modulating heating and cooling valve actuation. Rishel""s recommendation is contained in an article that appeared in the November 1998 issue of the ASHRAE JOURNAL. Many controllers employ a limited range of position command signal calculations. For example, Siemens xe2x80x9cMBCxe2x80x9d controllers which are widely employed for heating, ventilating and air conditioning systems have an allowable range of reposition intervals for the controller modules of from 1 to 15 seconds. This range of position command signal calculations is typical of many manufacturers and applications. A repositioning calculation nearly always results in a slightly different position command signal sent to the actuator of the valve or other device, because most calculations are quite complex and include a time based (integral) error signal as well as proportional error signal. The calculation in digital controllers is quite precise, usually accurate to at least one decimal place. Therefore, except for with an exceptionally stable application, a slightly new position command signal will be issued from the controller to the actuator that operates the valve or other device nearly every time the repositioning calculation is made.
When equipment must operate continuously and new position command signals occur once every second, the actuator will have been subject to more than 5,000,000 repositions in two months. If the positioning command signal is issued once every 15 seconds, the actuator will be subjected to more than 5,000,000 repositions in less than 30 months. Many electric actuators have a designed life expectancy of approximately 5,000,000 repositions. For example the Siemens model GDE/GLB Series actuator technical instructions 155-188P25 state that the actuator is designed for a life cycle of 5,000,000 repositions at rated torque and temperature.
Thus, when operated continuously and repositioned at typical intervals of from 1 to 15 seconds, such actuators have an expected life of between approximately 2 and 30 months. This short life expectancy range has been the cause of maintenance problems in certain applications. To increase the life expectancy, some actuators have incorporated logic that causes position command signals to be ignored if the difference between the current actuator position and the new position command signal is below a certain threshold value. This approach reduces the ultimate precision of actuation in order to increase the life span of the electric actuatorxe2x80x94for example, if an actuator is 50% open and the prior art threshold value is 5%. If the command changes to 53%, the valve will not be repositioned. If successive commands remain at 53%, the valve will not move no matter how long the command remains at 53%. Thus, the precision of the valve suffers because of the constant difference between the valve position and the command value. Only when the signal moves up to more than 55% or less than 45% would the actuator move. It is the conflict between actuation precision and prolonged actuator life that the present invention is intended to solve.
One aspect of the present invention enables electronically actuated valves to operate reliably with prolonged life by achieving effective control with less-frequent reposition requirements. The present invention also encompasses a method for determining when to reposition an electronic actuator that enables less-frequent valve repositions when repositioning commands are small. By including a variable time interval and a new method of determining when a minimum threshold of change between repositions is met or exceeded, the expected life of the electronic valve actuator is extended.
Consistent with the present invention, a time interval is selected at which repositioning command signals will be received. When a position command signal is received for a valve, a check is made to determine if the absolute value of the sum of reposition commands since the last repositioning of the valve exceeds a preset threshold. If it does, the valve is repositioned according to the most recently received reposition command. If the absolute value of the sum does not exceed the threshold, the valve is not repositioned, but the value of the most recent reposition command is added to memory. At the next time interval, a next repositioning command is received. This next command is added to the prior repositioning command values stored in memory since the previous valve repositioning, and the analysis is repeated with the absolute value of this new sum. After the valve is repositioned, the sum of reposition commands is reset to zero.
Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.